Gayston manufactures pressure vessels which hold pressurized stored gases including carbon dioxide (“CO2”), nitrogen and air. These pressurized gases are used in a variety of applications to operate sports equipment (ex: in paintball markers to propel the paintballs) and construction tools (ex: in portable power nailers), and in other applications where controlled bursts of pressurized gas are used to provide energy to operate a mechanism.
In the interest of efficient, safe and convenient use of products powered by pressurized gas, the user needs to be able to monitor the quantity of remaining gas which may be dispensed from the vessel before it needs to be re-filled or replaced. A pressure vessel containing nitrogen or air is filled until the gas in the vessel reaches a predetermined pressure based on the vessel's pressure rating. Then, as the gas is depleted, the pressure in the vessel drops steadily until all pressurized gas has left the vessel. The level of gas remaining in the vessel during use can therefore be determined by monitoring the pressure level of the gas remaining in the vessel as compared to its full pressure reading using a pressure gauge.
A pressure vessel containing CO2, however, is filled not to the vessel's pressure rating, but to a predetermined weight of CO2 based on the vessel weight capacity rating. This is because the pressure in a pressure vessel filled with CO2 will be the same for a given CO2 temperature regardless of the vessel's level of CO2 fill. For example, two pressure vessels filled with CO2, one to 20% of capacity and one to 80% capacity, will both have an internal pressure of about 860 PSI at 68F. This behavior of CO2 occurs because pressurized CO2 exists simultaneously in both liquid and gas phases across a wide range of operating pressures and temperatures. When CO2 is depleted from the vessel during use, additional liquid CO2 inside the vessel “boils off” to form more gas CO2 until the equilibrium pressure for the CO2 in the vessel at its temperature is reached. This constant pressure-at-temperature characteristic for CO2 continues until all of the liquid CO2 has “boiled off,” after which the vessel will only contain gas phase CO2. The gas phase CO2 pressure will then gradually be reduced as the remaining gas is depleted (as is the case for air and nitrogen gases as noted above). Consequently, the use of a pressure gauge to monitor the CO2 remaining in a pressure vessel does not provide an accurate indication of fill level until all of the liquid phase CO2 has been depleted, after which the pressure in the vessel finally begins to drop proportionate to the use of the remaining gas phase CO2. This occurs only when the vessel is nearly empty.
For example, the pressure vessels used in paintball markers typically contain all gas phase CO2 at about 10% or less of capacity. In this example, a pressure gauge will only accurately read the final 10% or less of CO2 depletion. A paintball player with less than 10% CO2 has to become very conservative with shots to avoid running out of pressurized gas. Furthermore, it would be advantageous for paintball players to be able to check the level of CO2 remaining in a previously-used pressure vessel before going to a paintball park with a mostly depleted pressure vessel. Current pressure gages used on similar pressure vessels are rendered effectively useless for these paintball players. It would be desirable to solve these problems described and determine the liquid level of CO2 contained within a pressurized vessel, particularly a pressurized vessel used for paintball applications.